1. Field of the Invention
The present invention relates generally to the field of digital radio signal communications. More particularly, the invention relates to transmitting a common pilot channel using an antenna array that is also employed as a beamforming antenna array, such as a spatial diversity antenna system.
2. Description of the Related Art
Many wireless data communication systems use training information or pilot signals that the receiving terminal uses to demodulate received traffic, control, overhead or other signals. One such example is the pilot signal transmitted by a base station in a CDMA (code division multiple access) communications system. Typically a single common pilot signal is transmitted to all remote or subscriber terminals in the base station's coverage area. The coverage area of the base station is typically referred to as a sector. Any signals transmitted to any particular user terminal can then be resolved with the help of the timing and phase information in the common pilot signal.
The pilot signal is particularly effective when the signal propagation path for both the pilot and the user-specific signal is the same. In a CDMA system in which the pilot and user-specific signal are sent over the same frequency band from the same antennas but with different scrambling codes, the pilot signal is very effective. However, any difference between signals makes the pilot signal more difficult to use or, in other words, it makes the user-specific signal more difficult to demodulate. If, for example, the user specific signal is spatially directed toward the remote terminal and the pilot signal is a common sector-wide signal, then the two signals can traverse a different signal propagation path. This will cause the two signals, as received by the remote terminal, to differ.
The propagation channel of the pilot signal and the propagation channel of the traffic channel will differ whenever they are transmitted with beams of different width and shape. The user terminal typically uses the pilot signal to estimate a channel that then is used in the process of demodulating and detecting the symbols transmitted on the traffic channel. The difference in the propagation channel of the pilot signal and the traffic channel therefore reduces the accuracy of the channel estimate. To compensate, each user can be provided with a user-specific pilot signal but this greatly increases the amount of traffic on the network.